The present invention relates to filters, and particularly to the multiple-disc type filter such as are now widely used in filtering particles from irrigation water and in many industrial and other applications.
The multiple-disc type filter includes a housing in which the filter body within the housing is in the form of a stack of like, centrally-apertured, filter discs of substantially uniform thickness along their widths and having grooved side faces defining filtering channels between the adjacent discs in the stack. In some applications of such filters, the outer face of the stack of filter discs constitutes the upstream side of the filter, in which case the fluid being filtered passes from the outer face to the inner face of the stack; and in other applications of such filters, the inner face of the stack constitutes the upstream side of the filter, in which case the fluid being filtered passes from the inner to the outer face through the filter stack.
Multiple-disc type filters have a number of advantages over other known types of filters, for example, the cylindrical-screen type filter. Thus, the multiple-disc filter has a larger capacity for removing and retaining dirt particles since the dirt particles may be retained also between the side faces of the discs, in addition to being retained on the upstream surface as in the cylindrical-screen type filter. Another advantage in the multiple-disc filter is that it is not as easily ruptured as the screen type and therefore there is less danger that because of a malfunction, unfiltered water may pass through and clog sprinklers or other devices downstream of the filter. The latter advantage is particularly important in self-cleaning filters wherein the upstream face of the filter is cleaned by a cleaning nozzle which, in the case of a screen-type filter, may rupture the screen by particles becoming wedged between the cleaning nozzle and the filter screen.
However, one of the disadvantages of the multiple-disc type filter is that, although the filter discs are of substantially uniform thickness, there is nevertheless some variation, because of manufacturing tolerances, between the discs, and these variations accumulate in the stack. Thus, a variation of a mere 0.05 mm. in the thickness of one disc, although within a reasonable manufacturing tolerance for an individual disc, becomes problematical when this variation is multiplied by the number of discs, e.g., 150 discs, in the stack. Such thickness variations when multiplied by the number of discs in the stack may result in very significant non-uniformities in spacings between the discs, and therefore non-uniform filter channels. Moreover, since the fluid being filtered seeks the path of least resistance when flowing through the filter body, these non-uniformities in the disc spacings may cause "streaming" therethrough, thereby actually bypassing the filtering channels.